Controller initialize and loadComponent()

Add/update docs for Controller::initialize(), addComponent() -> loadComponent(), addBehavior() -> loadBehavior().

Waiting on cakephp/cakephp#4635
I always felt that this should be more prominent. May be as a note. Or an **additional** recap-in-a -sentence-note.
Something like:

>  ``AppController`` property values are merged with child controller class arrays and the child class will always override those in ``AppController.``